When a well such as an oil and gas well produces a liquid product such as crude oil which is desirably recovered at the earth's surface but the well does not have sufficient natural pressure in the reservoir to force the liquid product to the earth's surface, artificial pumping, sometimes referred to as artificial lifting, is employed. Artificial pumping forces the desired liquid product through the wellbore to the earth's surface for recovery and other utilization as desired.
One of the more common forms of artificial pumping used, particularly in the oil patch, is sucker rod pumping wherein a sucker rod string is employed in the wellbore and carries at the bottom end thereof a positive displacement pump. By reciprocating the sucker rod string and pump up and down inside the well tubing, work is transferred to the downhole positive displacement pump with the result that liquid product in the well tubing is forced upwardly through the tubing to the earth's surface.
The sucker rod string is reciprocated through the use of a pumping unit located at the earth's surface. This requires the sucker rod string to be reciprocated continuously through an opening in the wellhead that caps the well at the earth's surface. The area of concern to which this invention is directed is the area where the sucker rod string enters the wellhead. A seal is universally employed in this area to prevent well fluids, both liquid and gaseous, from escaping from around the sucker rod string out of the wellhead and into the earth's atmosphere.
The type of sucker rod seal most used by the prior art employs packing elements that are mechanically forced against a highly smooth sucker rod section called a polish rod. The packing elements press against the polish rod to achieve good sealing around the polish rod and to prevent, to the maximum extent possible, leakage from the interior of the wellhead around the polish rod and then to the exterior of the wellhead. Because the polish rod moves repeatedly up and down through these packing elements over a long period of time, the elements are subject to mechanical wear so that the stuffing box requires continual monitoring to adjust or replace worn packing elements to insure leakage prevention.
This type of stuffing box and its packing elements rely on the presence of some liquid lubricant between the polish rod and the packing elements which can sometimes escape outside the wellhead. Further, this equipment is not designed to form a reliable gas-tight seal around the polish rod so that the escape of volatile organic compounds from inside the wellhead and around the polish rod is also possible. As requirements tighten on the tolerable amount of liquid and gas allowed to escape around the polish rod, operator monitoring of each stuffing box necessarily increases, sometimes even to daily checks, to insure compliance with the volatile organic compound emission limits.